Fermi National Laboratory

Fermilab Result of the Year 2004

On January 11, the Theory Group postdocs gathered at the Users' Center to discuss the most outstanding experimental result from the 2004 Fermilab Results of the Week. Fortified with a round of beer, the postdocs settled in for a vigorous debate. Surprisingly, a clear and unanimous choice appeared in the first round of suggestions. The outstanding result of 2004 was the precise measurement of K-long branching fractions and extraction of V_us by the KTeV Collaboration (June 10).

The new measurement of V_us by the KTeV Collaboration solved a mini-unitarity crisis in the CKM matrix. Previously there was a 2.2 sigma discrepancy between the PDG world average of V_us measurements, and the unitarity constraint. By measuring the six largest K-long decays in the same experiment, the KTeV Collaboration was able to significantly reduce the overall uncertainty in the branching fractions, and found a V_us that agrees within 1 sigma of the prediction of unitarity.

Having come to the decision so quickly, it was decided to order another round, and to argue over the runners up. The predominant choice for second place was the precise measurement of the charmed bottom meson B_c mass, reported by both DZero and CDF (December 2 and 3). Not only were these impressive results, but they were also the first clear test of a lattice QCD prediction made before the corresponding measurement had taken place. The lattice QCD prediction was spearheaded by the Fermilab Lattice Collaboration, along with the HPQCD and UKQCD Collaborations, and was released two weeks before the measurements were announced.

Three measurements vied for third place. They competed not only on the beauty of the measurements, but in their significance to the general physics program. Ultimately the measurement of a Z boson decaying to two tau leptons won out (August 19 result of the week by DZero, and similar measurements by both CDF and DZero). In most Run II analyses a tau lepton is treated as another jet, because of its mostly hadronic decays. The ability to identify taus as leptons opens up the possibility for observing several new signals for Higgs and supersymmetry. Polarization information in tau decays may provide additional handles on the extraction of clean signals.

The two runners up were the first measurement of the Z plus b jet cross section at a hadron collider (September 30), and the measurement of the angular correlation of dijets (May 13). While the Z plus b jet cross section is relevant in the search for a Higgs boson, it is currently more important in understanding details of heavy-quark parton distribution functions, initial-state radiation with heavy quarks, and aspects of perturbative QCD. The dijet angular dependence is both an important check on QCD predictions at hadron colliders, and is a severe test of the Monte Carlo tools used to compare experiment to theory.

by Zack Sullivan

The members of the 2004 Result of the Year committee are: Ayres Freitas, Ulrich Haisch, Jack Laiho, Olga Mena, Masataka Okamoto, Jose Santiago, Peter Skands, Zack Sullivan, and Giulia Zanderighi.

last modified 2/21/2005   email Fermilab